1. Field of the Invention
This invention relates to a method and apparatus for measuring the effectiveness of support systems used to reinforce an underground excavation and, more particularly, to a system for monitoring the roof control system utilized in a longwall mining operation.
2. Description of the Prior Art
In underground mining, excavation and tunneling operations, it is conventional practice to reinforce the exposed overhead and lateral rock strata by support systems. The support systems may include conventional wood timbering, cribs, and concrete cribs. Elongated anchor bolts are inserted in bore holes drilled in the exposed rock strata. The anchor bolts are anchored in the bore holes by mechanical expansion shells, resin, or a combination of both, as illustrated in U.S. Pat. No. 4,865,489, and are tensioned to compress a bearing plate against the rock strata. Anchor bolts are also used to secure metal roof mats and channels across a mine roof and downwardly along the lateral sidewalls or ribs of an entry. The mats and channels are provided in various lengths with holes spaced a preselected distance apart through which roof bolts extend and are anchored in the strata to maintain the channels compressed against the surface of the rock strata.
Another known underground support system is the truss-type support, as disclosed in U.S. Pat. Nos. 4,601,616 and 4,934,873, which includes one or more rods connected and extending horizontally the width of a mine passageway. The rods are connected at their ends to anchor bolts which extend at an angle adjacent the ribs of the passageway into the rock strata over a solid pillar. The rods are tensioned so that the vertical components of the compressive forces are transmitted into the solid material over the pillars.
In underground mining operations, a wide variety of roof support requirements are encountered necessitating the use of many of the above-described support systems. In some applications, wood timbering and cribbing are cost effective and provide adequate support. In other applications, mechanical and resin bonded roof bolts are used primarily because of their ease of installation, cost effectiveness and superior anchorage. In certain applications, the use of channels and mats is preferred. In conditions where roof bolts and/or channels and mats are not totally effective, the truss-type roof supports are commonly used.
A longwall mining operation is an example of an underground mining system in which a wide variety of devices are used to reinforce the excavated areas beneath the rock strata. In a conventional longwall mining operation, a panel is developed for extraction or recovery of the mine material. The panel typically has a transverse dimension of 600 to 800 feet formed by parallel spaced headgate and tailgate entries extending a considerable distance, for example 4,000 to 10,000 feet, into the seam of mine material.
The panel is initially formed using a continuous mining machine. The longitudinally extending entryways include at one side the tailgate entry and at the opposite side the headgate entry. The tailgate entry is used for ventilation purposes and also serves as a main escapeway for personnel working at or near the longwall face. Also in the event of an emergency occurring on the headgate side of the longwall panel, an escape route is provided off the longwall face through the tailgate entry to a main entry.
The headgate entry is also used to promote face ventilation and to convey the dislodged material from the working face to a series of sub-main entries where conveyors transport the mined material out of the mine. The headgate and tailgate entries are also accessed transversely for the movement of personnel and equipment to and from the longwall face from other longitudinally extending entries by cross cut or bleeder entries. Bleeder entries connect with the headgate and tailgate entries and serve to provide airflow to ventilate these areas and remove methane gas from the mine face.
At opposite transverse ends of the panel are located the set-up room and the recovery room. The longwall shearers, shield supports and pan line are assembled in the set-up room. In operation, the shearers transverse the panel face beneath the shield supports between the headgate and the tailgate entries. The dislodged material is conveyed by the pan line laterally to the headgate entry and therefrom out of the mine. The longwall mining operation continues until the shearers break through the panel into the recovery room.
When the longwall operation reaches the recovery room, the shearers, shield supports, and pan line are disassembled. The recovery room is connected to adjacent entries by recovery chutes and cross cuts leaving solid pillars in place to support the overhead structure. The disassembled shearers and pan line and retracted shield supports are moved out of the recovery room through the recovery chutes. The longwall shield supports are lowered from contact with the mine roof and advanced from the recovery room to the next location where the longwall mining machine is set up for extracting another panel.
The various excavated sections of a longwall mining operation require different types of roof supports. In certain entries, a primary system of mechanical roof bolts provides adequate overhead support; while, in other areas mats and channels are preferred. Certain roof conditions may require the utilization of a combination of mechanically anchored roof bolts and a truss system. Therefore, in a longwall mining operation particular attention must be given to the type of roof support used in the headgate entry, tailgate entry, set-up room, recovery room and access chutes to and, from the set-up and recovery rooms. It is important that the roof control system be installed so as to provide a safe working environment for personnel and equipment .and prevent interruption in the mining operation due to roof falls and pillar failures.
In a longwall mining operation, the recovery or teardown room is developed before the panel is extracted. This requires that the rock strata above the roof of the recovery room be supported to withstand the abutment pressures that are applied thereto when the longwall mining machine has advanced the panel closely adjacent to where it breaks through into the recovery room. The conventional method of supporting the roof of a recovery room includes cribbing, generally fabricated of wood or concrete, positioned adjacent to the wall of the recovery room where the longwall shearers break through and also adjacent the outby wall of the recovery room.
The span of the roof of the recovery room between the cribbing is conventionally supported by roof bolts. It is also known to use wire rope trusses and wire screening to support the mine roof to withstand the abutment pressures that are generated as the longwall shearers approach the termination line of the panel before the breakthrough into the recovery room. Even with these measures taken to support the mine roof, the abutment pressures can build to a magnitude causing failure of the pillar of material between the longwall shield supports and the recovery room before the shearers cut through the termination line into the recovery room.
When the roof immediately in front of the shield supports fails before the longwall shearers reach the recovery room, substantial delays in the mining operation are encountered. The material from the roof fall must be removed and thereafter the exposed roof must be reinforced before the shearers can be advanced into the recovery room. Various methods have been proposed to provide additional reinforcement of the roof above the recovery room to resist the abutment pressures generated by the advancing longwall so that the pressures are dissipated over the recovery room to the surrounding solid pillars. However, present methods, such as injecting polyurethane glue into the immediate roof in advance of the shield supports, constitute a substantial material cost and require interruption of the mining process to allow the glue to set, resulting in an .expensive loss of production. Installing wire meshing and bolting the exposed roof immediately in advance of the shield supports have not proved adequate to eliminate the exposure of hazardous conditions to personnel working beneath the roof in advance of the longwall shield supports. Furthermore, installation of wire meshing and roof bolts in advance of the shield supports near the termination line causes substantial delays in the longwall advance rate.
It is well known to monitor the behavior of rock strata surrounding an excavated area during a mining operation as disclosed in U.S. Pat. Nos. 3,600,938 and 4,136,556. U.S. Pat. Nos. 3,594,773 and 4,514,905 disclose extensometers extending between the mine roof and floor for measuring the roof to floor convergence. In the event the mine roof subsides a predetermined distance along the vertical, a signal is generated to warn of a danger of roof collapse. U.S. Pat. No. 4,913,499 discloses a convergence measuring device positioned on a longwall roof support.
U.S. Pat. No. 4,581,712 discloses a system for measuring and monitoring the stress levels applied to mine roof bolts and support columns. Sensors are positioned between the mine roof and portions of the roof bolts or support columns. The sensors are electrically connected to modules which transmit signals to a host computer when a sufficient change is detected in the load applied by the overhead rock strata to the roof bolt. In this manner, a detection in the change of the load on the roof bolt provides a warning of a possible cave-in or roof-fall.
The use of extensometers and strain gages to analyze rock mass behavior is discussed in the article entitled "Evaluation of Cable Bolt Supports at the Homestake Mine" by J. M. Goris, F. Duan, and J. Pfarr, published in the March 1991 issue of CIM Bulletin. These instruments were used to determine the effectiveness of cable bolts for supporting rock masses during the mining operation. The extensometers were grouted into holes positioned adjacent to cable bolts installed in a preselected bolt pattern to support the rock masses. Strain gages were attached to the cable bolts. Data from the instruments provided an indication of the effectiveness of cable bolts to support the rock masses.
It is also known, as disclosed in U.S. Pat. Nos. 4,453,846; 4,887,935; and 5,029,943, to monitor and collect data transmitted from longwall roof supports relating to the operation of the roof supports.
While it is known to use measuring devices in an underground mine to monitor the stability of rock formations reinforced by roof support devices and to monitor the movement of mining equipment in the mine, there is a need for an instrumentation plan that monitors the effectiveness of a roof support system installed throughout a longwall mine. Such an instrumentation plan must be capable of recording the load pressures exerted on the roof support device as the longwall mining operation progresses in order to identify the areas of maximum pressure in the mining operation.